Missouri’s Patients Bill of Rights is three years old now but it’s once again the subject of debate. President Clinton plans a visit to Columbia Thursday to applaud Missouri’s H.M.O. bill. Clinton is expected to use the legislation authored by Representative Tim Harlan of Columbia in 1997 to fuel his argument for a national bill. Missouri’s version makes H.M.O.’s potentially liable when something goes wrong–a major point of partisan debate in Congress. It also lets patients appeal if they have been denied coverage and makes it easier to get into an emergency room.